System and method for attaching optical fibers to chips

ABSTRACT

A device for attaching at least one optical fiber to a chip includes at least one nanowaveguide disposed on a substrate of a chip to be attached to an at least one off-chip fiber respectively. At least one oxide taper mode converter is disposed around a nanowaveguide end and in optical communication with and modally coupled to each of the at least one nanowaveguide respectively, and adapted such that each corresponding fiber of at least one off-chip fiber corresponds to a cleaved fiber end each cleaved fiber end to be fused to each oxide taper mode converter respectively to optically couple and mode match each cleaved fiber end to each of the nanowaveguide ends of each of the at least one nanowaveguide via the oxide taper mode converter by a modal coupling. A method for attaching at least one optical fiber to a chip is also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a CIP of co-pending U.S. patent application Ser. No.16/479,078, SYSTEM AND METHOD FOR ATTACHING OPTICAL FIBERS TO CHIPS,filed Jul. 18, 2019. The Ser. No. 16/479,078 application is a nationalstage application under 35 U.S.C. § 371 of PCT Application No.PCT/US2018/014162, filed Jan. 18, 2018, SYSTEM AND METHOD FOR ATTACHINGOPTICAL FIBERS TO CHIPS, published as WO 2018/136597 A1, which claimspriority to and the benefit of U.S. provisional patent application Ser.No. 62/448,470, SYSTEM AND METHOD FOR ATTACHING OPTICAL FIBERS TO CHIPS,filed Jan. 20, 2017, all of which applications are incorporated hereinby reference in their entirety.

FIELD OF THE APPLICATION

The application relates to coupling optical fibers to nanowaveguides ofa chip, more particularly to coupling at least one optical fiber to ananowaveguide of a chip.

BACKGROUND

Integrated circuits and the chips within (e.g. integrated chips, oroptical portions of chips) typically communicate by light with otheroptical components via one or more optical fibers. In the prior art,fibers can be coupled to one or more optical connection points of a chipfrom a cleaved end of a fiber via an intermediate lens (e.g. a microlens), or via a shaped fiber end lens of the connecting fiber itself.For reasonably low loss connections (i.e. best light transfer across theconnection), tight alignment tolerances usually require active alignmenttechniques. Active alignment techniques are those which pass lightthrough the fiber and connection to literally measure the efficiency oflight transmission across the connection.

SUMMARY

A device for attaching at least one optical fiber to a chip includes atleast one nanowaveguide disposed on a substrate of a chip to be attachedto an at least one off-chip fiber respectively. Each nanowaveguide ofthe at least one nanowaveguide has a nanowaveguide end. At least oneoxide taper mode converter is disposed around the nanowaveguide end andin optical communication with and modally coupled to each of the atleast one nanowaveguide respectively, and adapted such that eachcorresponding fiber of at least one off-chip fiber corresponds to acleaved fiber end. Each cleaved fiber end is fused to each oxide tapermode converter respectively to optically couple and mode match eachcleaved fiber end to each of the nanowaveguide ends of each of the atleast one nanowaveguide via the oxide taper mode converter by a modalcoupling.

The at least one off-chip fiber can be optically coupled to a connectorat an end of the at least one off-chip fiber opposite to the chip. Achip can thus be provided in a box or package with at least one opticalconnector as a plug connectable or a plug replaceable component.

The nanowaveguide end can further include a tapered nanowaveguide end.The oxide taper mode converter can include a horn taper or an inversehorn taper. The oxide taper mode converter can include a straight taper.The oxide taper mode converter taper can include a selected one of: alinear taper, a hyperbolic tangent taper, an elliptical taper, anexponential taper, and a s-bend taper.

A portion of the substrate adjacent to the at least one oxide taper modeconverter can be removed to further optically isolate the oxide tapermode converter from the substrate.

At least one of the at least one off-chip fiber can include a circularcross section. At least one of the at least one off-chip fiber caninclude a square or rectangular cross section.

At least one of the at least one nanowaveguide can include a rectangularor trapezoidal cross section.

The cleaved fiber end further can include a fiber end taper.

A method for attaching at least one optical fiber to a chip includingthe steps of: providing at least one nanowaveguide of a chip includingat least one nanowaveguide end to be attached to at least one off-chipfiber respectively; forming at least one oxide taper mode converter overor adjacent to, and around each of the at least one nanowaveguide endand creating at least one taper on an outer surface of each of the atleast one oxide taper; cleaving at least one fiber end of at least oneoptical fiber, each cleaved fiber end of the at least one fiber endcorresponding to each oxide taper mode converter of the at least oneoxide taper mode converter respectively; aligning the chip so that anend face of each of the at least one oxide taper mode converter ismechanically aligned substantially adjacent to each correspondingcleaved fiber end; and fusing each of the at least one oxide taper modeconverter with each of the at least one fiber end respectively tomodally couple each of the nanowaveguides to each of the at least onefiber end via each of the oxide tapers.

The step of providing, can include the step of tapering an end of atleast one nanowaveguide of a chip to be attached to at least oneoff-chip fiber respectively, to create at least one taperednanowaveguide end.

The step of forming at least one oxide taper mode converter over oradjacent to, and around each of the at least one nanowaveguide end, andcreating at least one oxide taper, can include creating a horn taper oran inverse horn taper. The step of forming at least one oxide taper modeconverter over or adjacent to each of the at least one nanowaveguide endand creating at least one oxide taper, can include creating a straighttaper.

The step of tapering an end of at least one nanowaveguide can furtherinclude removing a portion of a substrate of the chip adjacent to atleast a portion of each nanowaveguide tapered end, or adjacent to eachtapered oxide end.

The step of fusing each of the at least one oxide taper mode converterwith each of the at least one fiber end respectively can include furtheralignment of each fused modal coupling by a surface tension of a heatedmaterial of both of the oxide taper mode converter and the fiber end.

Immediately before the step of cleaving each tapered fiber end, therecan be a step of forming a taper on an end of each fiber of at least oneoff-chip fiber, to form at least one tapered fiber end.

A device for attaching at least one optical fiber to a chip includes atleast one nanowaveguide disposed on a substrate of a chip to be attachedto an at least one off-chip fiber respectively. Each nanowaveguide ofthe at least one nanowaveguide has a nanowaveguide end. A chip substrateincluding a substrate undercut section is defined by a removed sectionof the chip substrate adjacent to an end of the nanowaveguide end. Thenanowaveguide end and the substrate undercut section are adapted suchthat each corresponding fiber of at least one off-chip fiber correspondsto a cleaved fiber end. Each cleaved fiber end is fused to an oxidetaper mode converter around each nanowaveguide end respectively tooptically couple and mode match each cleaved fiber end to each of thenanowaveguide ends of each of the at least one nanowaveguide inisolation from the substrate.

The foregoing and other aspects, features, and advantages of theapplication will become more apparent from the following description andfrom the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the application can be better understood with referenceto the drawings described below, and the claims. The drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles described herein. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1A shows a block diagram of a chip with exemplary opticalnanowaveguide connections;

FIG. 1B shows a drawing of a top view of an exemplary oxide taper andcorresponding tapered optical fiber; and

FIG. 2 shows a process diagram for a method for attaching at least oneoptical fiber to a chip in an absence of active alignment;

FIG. 3 shows a drawing of an exemplary optical fiber with a tapered endprior to fusing the tapered end to an oxide taper;

FIG. 4 shows a drawing of a view from above of the optical fiber andoxide taper of FIG. 3 after both are joined by fusing; and

FIG. 5 shows a drawing illustrating a schematic view of the fusedoptical coupling of FIG. 4 ;

FIG. 6A shows a drawing of a top view of another exemplary optical chipwith tapered nanowaveguides, oxide tapers and corresponding cleavedoptical fibers;

FIG. 6B shows a drawing of a side view of the oxide taper modallycoupled optical fibers and nanowaveguides of FIG. 6A;

FIG. 7 is a graph showing a coupling loss per facet;

FIG. 8A shows a top view of another exemplary optical chip with taperednanowaveguides;

FIG. 8B shows a side view of the embodiment of FIG. 8A with an undercutsubstrate region;

FIG. 9 shows a schematic diagram of an exemplary machine apparatussuitable for performing the new method of the Application;

FIG. 10 shows a drawing of an exemplary machine apparatus according toFIG. 9 ; and

FIG. 11 is a drawing showing an exemplary PIC with fused optical fibersaccording to the new device for attaching at least one optical fiber toa chip of the Application with optical connectors.

DETAILED DESCRIPTION

In the description, other than the bolded paragraph numbers, non-boldedsquare brackets (“[ ]”) refer to the citations listed hereinbelow.

Definitions

The word “around” is intended to have the common English meaning intechnical writing of all sides of, to encircle.

As described hereinabove, integrated circuits and the chips within (e.g.integrated chips, or integrated optical portions of chips) typicallycommunicate by light with other optical components via one or moreoptical fibers. In the prior art, fibers can be coupled to one or moreoptical connection points of a chip from a cleaved end of a fiber via anintermediate lens (e.g. a micro lens), or via a shaped fiber end lens ofthe connecting fiber itself. For reasonably low loss connections (i.e.best light transfer across the connection), tight alignment tolerancesusually require active alignment techniques. Active alignment techniquesare those which pass light through the fiber and connection to literallymeasure the transmission efficiency of light transmission across theconnection.

As multiple input and/or output optical lines become more commonplace,new techniques are needed to attach multiple fibers to a chip withoutneed for active alignment. Passive alignment refers to coupling a fiberto an optical connection point by only a physical alignment of the fiberwith the connection point. In passive alignment, it is not necessary topass light through the fiber waveguide and to directly measure lighttransmission levels and/or optical alignment past the coupled opticaljunction.

Especially where multiple fibers are to be attached to a chip in highvolume production, what is needed is a new device and method for passivealignment of the connecting fibers during mass production.

Techniques of fusion by electrodes or laser of discrete individualfibers are well known. The discrete individual fibers are typicallyfibers of a multi-fiber cable, or equipment interconnecting singlefibers, such as single fiber cables with an optomechanical connector oneither end. Usually such techniques make use of a fusion apparatus witha microscope to initially mechanically align two cleaved fibers to bejoined together, such as to make an inline optical splice. For example,as described in Corning, Application Note AN103, Single Fiber FusionSplicing, such methods, especially when combined with active alignment,can provide splices with losses as low as in the range of 0.05 to 0.10dB for both single-mode and multimode fibers. Also, as described in theAN103 note, fibers naturally tend to self-align due to surface tensionof the melted glass. Emergency splicing techniques for discreteindividual fibers are based on passive aligned fibers where single-modefiber splice losses between 0.5 to 1.5 dB usually can still be obtainedby only mechanical alignment combined with self-alignment of joinedmelted fiber ends.

Unfortunately, the chip waveguide (e.g. nanowaveguide) ends have beentoo small compared to a typical connecting fiber to make a prior arttype fusion connection as an efficient (low loss) optical coupling to anexternally connected fiber. In fact, the nanowaveguides of today'stypical chips can be hundreds of nanometers across, nearing thewavelength of light. By contrast, connecting optical fibers aretypically at least in the um diameter or cross section range. Well knownfusion techniques of the prior art for splicing two similar diameter orcross section fibers together cannot be used.

It was realized that by overlaying an oxide layer on a tapered end ofeach chip nano waveguide, or by placing an intermediate oxide taperbetween a tapered end of each chip nano waveguide and an opticallycoupled fiber, that the fiber, with or without a tapered fiber end, canbe joined to each oxide taper to create a low loss optical connection byuse of passive alignment of the fiber with the oxide taper alone.Moreover, where in production, the mechanical location of each of suchnew oxide tapers can be known with substantial accuracy with the respectto one or more locations of the chip. Therefore, it becomes possible tomake one or more fiber connections to chip by any suitablemicrofabrication technique, such as, for example, by parallelfabrication methods or by step and repeat methods, by mechanicallyalignment alone (passive methods) followed by fusion by any suitablemethod. Known fusion methods such as, for example, by electrode(electrical arc) or laser can be used. Just as in the Corning emergencyfusion method for splicing two similar fibers, the quality of theoptical connections can be enhanced by self-alignment of the oxide taperand fiber material (e.g. glass) due to surface tension of the meltedmaterials.

Merely expanding the end of the chip waveguide, while possibly providinga sufficiently large end cross section for a fusion splice, would notwork because of disruption of the typical single mode transmission oflight to or from the chip. It was realized that by further includingcertain types of tapers formed both on the end of the chip waveguide andwithin a corresponding taper on an outer surface of the oxide taper,there can be made a more efficient optical coupling to a relativelylarge physical cross section of a chip connection point. However, eventhe relatively large physical cross section of the chip connection pointis still relatively small with respect to the diameter or width of anend of a fiber to be coupled. In some embodiments, the difference is toolarge to support or provide an optical low loss connection. Therefore,the solution can also include providing a taper at the end of theoptical fiber to be fusion coupled to the end cross section or face ofthe chip oxide connection taper connection point. By use of the newsystem and method for attaching optical fibers to chips without need foractive alignment described hereinabove, at least one off-chip fiber canbe efficiently coupled to chips in mass production with typical losseswell below about 1 dB, and optimally below about 0.2 dB. In someembodiments, the substrate can be removed (e.g. a silicon substrateremoved in the vicinity of the coupling location). A non-tapered fibercan also be coupled to the oxide taper.

FIG. 1A shows a diagram including an exemplary box “A” of the chip 1030where the optical circuitry has been provided on a substrate 1005, suchas by well-known integrated circuit and chip manufacturing techniques. Ananowaveguide 1003 a is to be terminated by optical coupling to anexternal fiber at an optical connection point show within box “A”.

FIG. 1B shows a diagram of one embodiment of the new system and methodfor attaching optical fibers to chips without need for active alignmentwhere on the left side of the drawing the end of waveguide 1003 a has amanufactured taper (tapered nanowaveguide end 1003 b) and overlaying(e.g. cylindrically, square, or rectangular symmetric, or planar withtapers only in the plane of the planar layer) oxide taper which matchesthe tapered nanowaveguide end 1003 b within, and provides an oxide taper1006 of the oxide optical connection point at location 1017. To theright of the expanded section “A” in FIG. 1B, the end of tapered end1021 to be optically coupled, and typically, mechanically fused to oxideoverlay 1001 can be seen to taper to a similar diameter or cross-sectiondimension of the end face connection point of the oxide taper 1006.While in the exemplary embodiment of FIG. 1B, the oxide taper matchesthe tapered nanowaveguide end, it is not necessary to match the shape,though matching the shape can slightly improve coupling efficiency. Forexample, a circular fiber can be mechanically (e.g. fused) and opticallycoupled to a rectangular or square oxide taper.

Light can be efficiently coupled from a tapered nanowaveguide end 1003 bto the oxide overlay 1001 by modal overlap. Geometry, width, angles, andthickness of both oxide taper and waveguide nano-taper can be set tomaximize the modal overlap. For example, there could be a silicon taperof a thickness of on the order or 100 nm. The mode of the waveguidetaper matches the mode of the oxide taper such that light transfers byefficient modal coupling.

Tapers of nanowaveguide 1003 a (tapered nanowaveguide end 1003 b), oxideoverlay 1001 (having oxide taper 1006), and tapered end 1021 of opticalfiber 1023 can be made by any suitable means or production technique nowknown or future developed. For example, the waveguide nano-taper can bemade using techniques of photolithography, electron beam lithography,and/or other nano manufacturing techniques. The tapered end 1021 ofoptical fiber 1023 can be formed, for example, by heating and drawing,laser ablation, or mechanical grinding or ablation. Suitable techniquesinclude, for example, laser with mechanical pull, electrical arc withmechanical pull, flame techniques (e.g. acetylene flame) with mechanicalpull, etc. Ablation, such as, for example, by laser ablation, can alsobe used.

The fiber is typically cleaved by any suitable technique, typically,after the taper is formed. Fiber cleaving techniques are well-known.

Similarly, the nanowaveguide 1003 a tapered nanowaveguide end 1003 b ofthe chip, and/or the oxide taper 1006 which overlays the taperednanowaveguide end 1003 b can be post process shaped by post processingsteps of lithography, or laser or mechanical ablation. The exact methodof forming any of the tapers is unimportant to the new system andmethod. Also, as described hereinbelow, the oxide taper canalternatively be adjacent to the tapered nanowaveguide ends as comparedto an overlay or partial overlay.

The exemplary embodiment of FIG. 1A shows at least a part of the oxidetaper as a structure extending past the edge of the substrate of chip1030. In other embodiments, there can be a “hanging structure”, forexample, by etching away a part of the substrate near to and/or under atleast one oxide overlay 1001. By so removing substrate in the vicinityof the coupling location, it can be easier (e.g. by creation somethermal and mechanical isolation near the coupling location) to createthe splice between the end of the oxide taper 1006 and the tapered end1021 where the two are to be mechanically fused at about location 1017.

Hanging structure: A hanging structure as used herein refers to an endof a nanowaveguide of a chip, where a portion of the substrate(typically under the nanowaveguide) near the end of the nanowaveguide tobe coupled off-chip, has been removed. The portion of the substrateremoved can be large enough to also clear, or partially clear, the oxidetaper overlaying the end of a nanowaveguide of a chip to be coupledoff-chip to a fiber. The hanging end of the nanowaveguide and oxidetaper combination to be coupled off-chip can be somewhat mechanicallyand thermally isolated from the substrate.

Tapered nanowaveguide end 1003 b can have a width ranging from about 50nm wide to a few microns depending on the material system and specificwavelength of light to be coupled to or from chip 1030. The oxide taperis can include a length range from about a micron to tens of microns.The width of the oxide taper can range from about 1 micron to about 150microns.

Alignment of oxide tapers and tapered ends of fibers: At least one ormore oxide tapers can be aligned to at least one or more tapered fibersend of an off-chip fiber to be connected by passive alignment methods.For example, there can be fiducials on the chip for alignment by avisual system, such as, for example, an assembly microscope. Opticalcoupling by fusing is typically improved by surface tension of thefusing glass which can pull the parts into a more optimal alignment.Also, with larger modes there could be a better tolerance tomisalignment. For example, the geometry of the oxide taper and taperedend of the fiber will make for a larger mode target for coupling ascompared to prior coupling methods. Even in early embodiments, lightlosses across the fused section can be at least between about 0.1 dB and1.0 dB. However, losses between about 0.01 dB and 0.1 dB can be achievedby the techniques described hereinabove by optimizing manufacturingalignment (still typically a passive alignment), materials (e.g. of theoxide taper), and dimensions, such as length width, and geometry of thetapers.

Tapers: Suitable tapers for the ends of the nanowaveguides, the oxidetapers, and/or the tapered fibered ends of the fibers include, forexample, linear tapers (e.g. FIG. 1B), hyperbolic tangent taper,exponential taper, and s-bend tapers. Typically, the goal is to make thetaper as short as practical combined with efficient light transfer (lowloss). While tapers can be symmetrical about a long axis of ananowaveguide, more typically as implemented in a planar layer of anintegrated circuit, there will be a taper in the integrated layer whichcan be seen, for example, as viewed from above normal to a substrate,and not as viewed from a side view or side cut-away view (i.e. as viewedsideways through the layer).

The taper on the chip side is typically manufactured by any suitablemicro-fabrication technique, such as, for example, knownphoto-lithography techniques.

Taper dimensions, and shapes: The efficiency of light transfer betweenthe fiber and the oxide taper typically depends on the relativedimensions of the fiber and the oxide taper. The dimensions can beoptimized to balance loss, sensitivity to misalignment, and size.

Oxide taper types: Suitable oxide taper types include straight, inversehorn taper, and horn. For example, for a tapered fiber, generally theinverse taper is preferred. For a cleaved fiber without a tapered fiberend, either of the horn taper or the inverse horn taper may bepreferred, generally with respect to mode coupling for the given modes.For example, where the mode goes from a smaller mode to a larger mode,or a larger mode to smaller mode. Also, as those skilled in the art willunderstand, either a horn taper (small to large, e.g. in the directionfrom the tapered waveguide end to the oxide taper element) or an inversehorn taper can be further defined by any suitable shape or function,such as, for example, a linear taper, a hyperbolic tangent taper, anelliptical taper, an exponential taper, or an s-bend taper.

The nanowaveguide of the new structure of the application propagates byradiated waves via end coupling, or butt coupling, from thenanowaveguide end via the oxide taper around the nanowaveguide into anoff chip optical fiber. Such coupling is distinguished from evanescentoptical coupling of the prior art, such as where planar layers overlap,usually only on one common side of a planar waveguide. The overlap insuch evanescent coupling is over a length larger than a cross sectiondimension of the waveguide (typically several cross section lengths ormore).

Moreover, the oxide taper as described hereinabove matches the mode ofthe optical fiber to the mode of the nanowaveguide, and thus the oxidetaper is an oxide taper mode converter. The tapered oxide section of thestructure of the Application optically couples and mode matches eachcleaved fiber end to each of the nanowaveguide ends of each of the atleast one nanowaveguide via the oxide taper by a modal coupling, as amode converter.

Tapered nanowaveguide end: While more typically the nanowaveguide willhave a tapered end, it has also been realized that the new method anddevise of the Application can be accomplished where a non-taperednanowaveguide (i.e. a nanowaveguide with a straight taper) is opticallyand modally coupled into an oxide taper (e.g. with a horn, straight, orinverse horn oxide taper) and on the opposite side of the oxide taperwhere the oxide taper is optically and modally coupled into a fiber(with or without a fiber tapered end). Where there is a taperednanowaveguide end, any suitable taper, including taper types and shapesdescribed for the oxide taper can be used as appropriate to the modalcoupling (e.g. if small to large, or large to small) between the taperednanowaveguide end and the oxide taper.

Example 1: In one exemplary embodiment, as illustrated by FIG. 3 , anoptical fiber 1023 is tapered down to tapered end 1021 having a circularcross-section 3020 of about 4 microns in diameter and an oxide taper1006 with a rectangular or trapezoidal cross section 3046 having aheight of about 4 microns, the optimum light transfer efficiency occursat a taper width of about 3.3 microns with a loss <0.1 dB. In thisexample, the misalignment tolerance is about 1.5 microns for about a 3dB loss threshold. Increasing the size of the mode by increasing thefiber 1023 and oxide taper 1006 dimensions improves the misalignmenttolerance almost proportionately. The dimensions of the waveguide taperdepend on the materials being used. For example, for a silicon waveguidetaper (this example), the silicon waveguide taper dimensions are betweenabout 120 nm and 170 nm.

Physical Cross sections: It is understood that while waveguide diameterand fiber diameter are used for convenience, there may be any suitablecross-section shapes. For example, typical chip waveguides are aboutrectangular, square, trapezoidal, or triangular in cross section shape.Typical connecting fibers are circular in cross section shape, howeveronce cleaved may have non-circular end face cross sections. In additionto the chip waveguide taper to oxide taper interface, the oxide taper1006 provides a physical interface with low loss mode coupling inaddition to a corresponding physical cross section end face fusioncoupling compatible with a cleaved and tapered end of optical fiber1023.

Square or rectangular cross section fibers, compared to more commoncircular fibers, may be especially suitable for fusing to square orrectangular oxide tapers according to the structures and techniquesdescribed hereinabove. For example, such square or rectangular shapes(including trapezoidal imperfections or distortions) are more common tochip fabrication. Therefore, modal coupling might be optimized in somemanufacturing scenarios by use of square or rectangular connectingfibers.

Materials: Exemplary materials systems suitable for optical fiberssilicon waveguide taper include silicon dioxide, glass, doped glass,silicon, and dichalcogenides. Typical materials for the oxide taper aredeposited or grown silicon dioxide, glass, doped glass, fluoride glasses(e.g. magnesium fluoride, calcium fluoride, etc.,) silicon nitride,semiconductor materials (e.g. silicon, indium phosphide, aluminumnitride, silicon carbide, gallium arsenide, etc.) which can bedeposited, grown or transferred (e.g. bonding). Typical materialssuitable for the waveguide taper include deposited or grown silicondioxide, glass, doped glass, fluoride glasses (e.g. magnesium fluoride,calcium fluoride, etc.,) silicon nitride, semiconductor materials (e.g.silicon, indium phosphide, aluminum nitride, silicon carbide, galliumarsenide, etc.) electro-optic or nonlinear materials (e.g. lithiumniobate) which can be deposited, grown or transferred (e.g. bonding).Typical substrates on which photonic devices are fabricated according tothe structures and techniques described herein, include silicon, fusedsilica, glass, quartz, sapphire, gallium arsenide, indium phosphide,silicon carbide, diamond, lithium niobate.

According to another exemplary embodiment of FIG. 3 , coupling of lightfrom an optical fiber to a silicon waveguide. An optical fiber 1023 istapered down to a tapered end 1021 having a diameter of about 4 microns.The fiber 1023 has a circular cross-section. The oxide taper 1006 isdesigned to match the mode of the tapered optical fiber 1023 withtapered end 1021 having a height (i.e. material thickness) of about 4microns and a width of about 3.3 microns.

As illustrated by FIG. 4 , the optical fiber tapered end 1021 and theoxide taper 1006 are fused at location 4015 according to the methodsdescribed hereinabove. The length of the oxide taper 1006 is optimizedto minimize optical loss. Finally, a silicon taper is used to couple thelight into the tapered nanowaveguide end 1003 b of a silicon waveguide,nanowaveguide 1003 a, having a height about 250 nm and width of about500 nm. The taper dimension is about 150 nm for optimal coupling to themode in the oxide taper, and the width of the waveguide taper slowlygrows to match the about 250 nm width of the waveguide.

FIG. 5 is a drawing illustrating another view of an optical fiber 1023fused to an oxide taper 1006 overlaying a tapered nanowaveguide end 1003b.

Example 3: A chip according to the structures and methods describedherein is typically fabricated by use of standard microfabricationtechniques known in the art, such as, for example, lithography, etching,deposition, planarization. In the exemplary embodiment of example 3, thewaveguide taper is lithographically defined together with the waveguideand covered in a material such as oxide. After the waveguidefabrication, the oxide taper is lithographically defined and etched intothe waveguide covering. The substrate is then removed if necessary, viaetching. The fiber is tapered by heating it close to its meltingtemperature and pulling until the desired diameter is reached. Theheating is accomplished by laser radiation, electrical arc, or flame.The tapered fiber is then cleaved using standard techniques, such asscoring the glass while applying tension. The tapered fiber and oxidetaper of the chip are passively aligned and fused together by meltingthe oxide at the interface of the tapered fiber and the oxide taper.

FIG. 2 shows the steps of a method for attaching one or more opticalfibers to a chip. The method includes the steps of: A) providing atleast one nanowaveguide of a chip including at least one nanowaveguideend to be attached to at least one off-chip fiber respectively; B)forming at least one oxide taper over or adjacent to each of the atleast one nanowaveguide end and creating at least one taper on an outersurface of each of the at least one oxide taper; C) cleaving at leastone fiber end of at least one optical fiber, each cleaved fiber end ofthe at least one fiber end corresponding to each oxide taper of the atleast one oxide taper respectively; D) aligning the chip so that an endface of each of the at least one oxide taper is mechanically alignedsubstantially adjacent to each corresponding cleaved fiber end; and E)fusing each of the at least one oxide taper with each of the at leastone fiber end respectively to modally couple each of the nanowaveguidesto each of the at least one fiber end via each of the oxide tapers.

Also, referring to FIG. 1A and FIG. 1B, a device for attaching at leastone optical fiber to a chip 1030 includes at least one nanowaveguide1003 a disposed on a substrate 1005 of a chip. Each nanowaveguide 1003 aof the at least one nanowaveguide has a tapered nanowaveguide end 1003b. The device also includes at least one oxide taper. Each oxide taper1006 of the at least one oxide taper is formed on an outer surface of atleast one oxide overlay 1001 and disposed over each of the taperednanowaveguide end 1003 b respectively. The device also includes a atleast one off-chip fiber. Each fiber 1023 of the at least one off-chipfiber has a corresponding tapered fiber end 1021. Each tapered fiber end1021 is fused to each oxide taper 1006 respectively to modally coupleeach tapered fiber end 1021 to each oxide taper 1006 by a modalcoupling.

Additional exemplary devices and methods for permanently attaching anoptical fiber to a photonic chip based on fusion splicing a cleavedoptical fiber to an oxide taper at the edge of the chip are nowdescribed.

Generally, in integrated optical devices, it is more efficient toprovide tapers during the formation of an optical layer. Therefore,those skilled in the art will appreciate in many embodiments, that thetaper of a so manufactured structure can be seen from above (top view,normal to the chip substrate), such as for example, the taper of atapered nanowaveguide of the chip, and/or the taper of the oxide tapertransition part disposed between the tapered nanowaveguide an off-chipcleaved optical fiber. Note also, that the phrase “off-chip” includesdiscrete interconnecting fibers, such as, for example, interconnectingbetween optically coupled chips within a common integrated circuitpackage (e.g. a hybrid integrated circuit with a two or more chipsdisposed within, such as, for example including an interposer chip), aswell as one or more optical fibers which extend to and from anintegrated circuit package.

While some embodiments hereinabove described a tapered nanowaveguide endextending into the intermediate oxide taper element, those skilled inthe art will appreciate variations of the same structures stillaccording to the method and device of the Application. For example,where the tapered nanowaveguide end and the oxide tapers are formed fromthe same or similar material compositions, following fabrication, thetapered nanowaveguide ends and the oxide taper elements may actually, orappear, to blend together, effectively not showing the taperednanowaveguide ends within the oxide taper elements. Relative insertiondimensions, where the tapered nanowaveguide ends extend into the oxidetaper elements can also depend on how much of the substrate is cutawayor cleared below and/or in the vicinity of the oxide taper. In someembodiments, the tapered nanowaveguide ends are not disposed within theoxide taper elements during manufacture and fabrication. For example, asshown in the exemplary embodiment of FIG. 6A and FIG. 6B, each of thetapered nanowaveguide ends of the nanowaveguides can be adjacent to andin in optical communication and modally coupled to each of the oxidetapers respectively.

FIG. 6A shows a drawing of a top view of another exemplary optical chipwith tapered nanowaveguides, undercut oxide tapers and correspondingcleaved optical fibers. A fusion spot from a laser is also shown. Thelaser melts the ends of the oxide taper and the optical fiber to fusethem together. The laser is aimed at the region to be fused, typicallyfrom the top or side of the chip being produced. FIG. 6B shows a drawingof a side view of the oxide taper modally coupled optical fibers andnanowaveguides of FIG. 6A. In FIG. 6B, the device has been laser fusedand the coupling interface optically sealed.

Silicon dioxide optical tapers can efficiently couple to an opticalfiber on one end, and to a waveguide nanotaper on the other opposite end(e.g. FIG. 6A, FIG. 6B). The geometry of the fiber side optimized formode matching with the optical fiber for optimum coupling between thefiber and the oxide taper. The geometry of the oxide taper has beenadiabatically varied to maximize the coupling into the waveguide taper.The device and method has been demonstrated to work with the differenttypes of inverse nanotapers (e.g. linear taper, metamaterial taper)because the geometry matches a desired mode profile.

As can be seen from the drawings of the Application, the oxide taper ofthe Application is around the nanowaveguide end, that is, the oxidetaper encircles the nanowaveguide end, including the embodiment of FIG.6B, where the nanowaveguide 6003 a butt couples to the oxide taper modeconverter 6006, which has a larger face area then the end of thenanowaveguide, where the oxide taper is around the end of thenanowaveguide.

Example 4: A device according to the Application used an oxide taper 9.5μm wide by 8 μm thick (FIG. 1 b ) to match a standard SMF 28 cleavedfiber. Due to the physical size mismatch between the fiber and the oxidetaper, the splice was reinforced with a UV curable optical adhesive. Therefractive index of the optical adhesive (such as, for example, aNorland optical adhesive, available from Norland products of Cranbury,N.J.) was used to further tailor the properties including mode size andshape of the oxide taper mode and improve the coupling to the fiber.

Photonic chips using the devices and methods described by theApplication can be fabricated using any suitable standard, typicallyCMOS compatible, microfabrication techniques.

Example 5: An about 4 μm thick layer of thermal oxide was grown on asilicon wafer and about 315 nm of silicon nitride was deposited via lowpressure chemical vapor deposition. The waveguides were patterned usingDUV optical lithography at about 254 nm using an ASML stepper. Afteretching in an inductively coupled reactive ion etcher (ICP-RIE) with aCHF₃/O₂ chemistry, the devices were clad with about 4 μm of oxide usingplasma enhanced chemical vapor deposition (PECVD). The oxide taper wasthen patterned and etched similar the waveguide step. After dicing, aportion of the silicon substrate was removed using a xenon difluoride(XeF₂) etch to optically isolate the oxide taper from the siliconsubstrate.

Generally, as described hereinabove in example 5, a portion of thesubstrate adjacent to the at least one oxide taper can be removed, orotherwise undercut, to further optically isolate the oxide taper fromthe substrate.

FIG. 7 is a graph showing the coupling loss per facet for the device ofexample 4. The graph shows a minimum loss of 3.1 dB at 1550 nm. Thecoupling loss of 3.1 dB was demonstrated per fact between a cleaved SMF28 optical fiber fused to a silicon nitride waveguide chip. The cleavedfiber was fused to the chip by heating the ends with a CO₂ laser. Oxidehas a strong absorption at a wavelength of about 10.6 μm and thesplicing leaves no residue behind. The laser beam was focused to a spotof about 45 μm using a ZnSe aspheric lens (f=20 mm), and aimed at thefiber-chip interface with a collinear red diode laser. The laser beamwas incident at an angle of about 30° to enable fusion of optical fiberarrays for future work. To fuse the fiber to the oxide taper, weirradiated the spot with 9 W of laser power for 1.5 seconds. The nitridewaveguide was about 1 μm wide and about 1.8 mm long nitride waveguidewith about a 0.25 μm wide by about 100 μm long tapers, which werecoupled to an about 50 μm long oxide taper that was about 11.5 μm wideat the waveguide side. Initially, we aligned the cleaved fibers to thechip and measure a 3.5 dB loss on each facet before splicing. Afterfusing the fibers to the tapers, the loss reduces to 3.12 dB per facet.The improvement in the loss shows that the fusion process does notdegrade the performance of the coupling.

Fiber to chip fusion splicing has the potential to enable highthroughput optical packaging with a robust, high efficiency, andlow-cost solution. This device and method can be fully automated toenable highly efficient fiber to chip coupling in high volumeapplications. The device and method is compatible with active andpassive (using fiducials on the chip) alignment. Calculations show thatthe coupling loss to at least about 1 dB per facet for cleaved fibersand down to about 0.2 dB per facet using tapered or narrow core fibers.

FIG. 8A shows a top view of another exemplary optical chip with taperednanowaveguides. FIG. 8B shows a side view of the embodiment of FIG. 8Awith an undercut substrate region. It was realized that in someembodiments, by adding an undercut alone of the substrate (typically asilicon substrate), that a fiber 8023 can be nearly optimally modallycoupled to a nanowaveguide taper 8003 b of a nanowaveguide 8003 a wherethere is added an undercut, such as an exemplary about rectangularundercut 8040, FIG. 8B. Note that in laser fusion, the laser spot isapplied directly to area to be laser fused, e.g. the lower jointmechanical coupling in FIG. 8A. To fuse the upper portion of FIG. 8A,the laser is redirected to point to that area, typically from a side orthe top of the structures to be fused together.

As in previous embodiments which add an optical adhesive (e.g. opticaladhesive 8050, FIG. 8B, it is noted that the fusion process (e.g. alaser fusion process) joins the optical components according to the newmethods and devices of the Application. The purpose of the opticaladhesive is not to “hold” parts together, but rather to give somemechanical protection against breakage. That is, the optical adhesiveprovides some mechanical support, in particular to protect against anangular deformation of the fiber with respect to the chip. As evidenceof the inherent strength of the mechanical coupling of the fusion jointsbetween optical fibers, typically where there is a break, the fractureoccurs at the chip, leaving the fused coupled mechanical parts intact.

FIG. 9 shows a schematic diagram of an exemplary machine apparatussuitable for performing the new method of the Application. FIG. 10 showsa drawing of an exemplary machine apparatus according to FIG. 9 . Theexemplary machine apparatus of FIG. 8 and FIG. 9 includes a CO2 lasercombined with a visible diode laser. The visible laser allows aiming ofthe CO2 laser, which is invisible to most imaging cameras. The CO2 beamis focused at the interface between the chip's oxide taper and theoptical fiber end. The visible laser can be used to find the focalposition of the beams. The chip and optical fiber can sit on translationstages to accurately align them.

In summary, the oxide taper can change the mode to bridge differentmodes between the optical fiber side of the oxide taper and the slightinsert or butt coupling to the nanowaveguide of the integrated circuit.That is, the oxide taper of the application which is around thenanowaveguide matches modes and can tailor the mode of light of theoptical fiber to a different mode of light more suitable for the planarnanowaveguide of the integrated circuit.

By so tailoring the modes, according to the new device for attaching atleast one optical fiber to a chip of the Application, the mode from theoptical fiber can be converted into a mode more suitable for thenanowaveguide, as opposed to the structures of the prior art, whichmerely couple the same mode from the optical fiber directly into thenanowaveguide (typically a tapered mode conversion) with lessefficiency.

Optical Fiber Connector

Using the structures and methods of the Application, a plurality ofoptical fibers can be efficiently optically coupled to a photonicintegrated circuit, such as including the fused fiber techniquesdescribed hereinabove.

However, in many applications it is more practical to manufacture orprovide a photonic integrated circuits based on the new device forattaching at least one optical fiber to a chip of the Application withone or more optical connectors.

FIG. 11 is a drawing showing an exemplary PIC 1100 with fused opticalfibers according to the new device for attaching at least one opticalfiber to a chip of the Application with optical connectors 1101. In theexemplary PIC with fused optical fibers of FIG. 11 , fibers are splicedfrom the connector to the chip and terminate at the package with aconnector 1101 so that the fibers can just be plugged into the package.As described hereinabove, we can fuse an array of fibers to the photonicchip using, for example, a CO₂ laser. The fused fibers have a connectoron the other end opposite the fused connections to the PIC. The PIC canbe packaged in any suitable box or package which contains the photonicchip, fibers and one or more connectors 1101. With or without anadditional package enclosure, a device as described hereinabove with atleast one connector (e.g. optical connectors 1101) provides a PIC (e.g.PIC 1100) which can be used a pluggable component.

The connector can be any suitable generic pluggable fiber connector. Anexample of a suitable fiber connector for at least one optical fiber isthe LC®/PC Single Mode Connector series, such as, for example, part no.30126L9 available from Thorlabs Inc. of Newton, N.J. Or, exemplaryconnectors for multiple optical fibers include, for example, the MTPConnector 12 Fiber MM available from Fiber Instrument Sales of Oriskany,N.Y., or the MTP-16 Short Spring Push Ribbon available from USCONEC ofHickory, N.C.

The addition of a connector provides a photonic device with pluggablefibers which eliminate the bunch of fibers hanging around in disarray aswell as providing a PIC based component with a pluggable installation orreplacement feature. That is, a chip with a device for attaching atleast one optical fiber to a chip according to the Application, can beprovided as a as plug connectable or plug replaceable component.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

REFERENCES

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[2] Hsin-Lun Tseng, Erik Chen, Haisheng Rong, and Neil Na,“High-performance silicon-on-insulator grating coupler with completelyvertical emission,” Opt. Express 23, 24433-24439 (2015).

[3] Martin Papes, Pavel Cheben, Daniel Benedikovic, Jens H. Schmid,James Pond, Robert Halir, Alejandro Ortega-Moñux, GonzaloWangüemert-Pérez, Winnie N. Ye, Dan-Xia Xu, Siegfried Janz, Milan Dado,and Vladimír Vaâinek, “Fiber-chip edge coupler with large mode size forsilicon photonic wire waveguides,” Opt. Express 24, 5026-5038 (2016).

[4] Barwicz, Tymon, et al. “A Novel Approach to Photonic PackagingLeveraging Existing High-Throughput Microelectronic Facilities.” IEEEJournal of Selected Topics in Quantum Electronics 22.6 (2016): 455-466.

[5] Pavel Cheben, Jens H. Schmid, Shurui Wang, Dan-Xia Xu, MartinVachon, Siegfried Janz, Jean Lapointe, Yves Painchaud, and Marie-JoséePicard, “Broadband polarization independent nanophotonic coupler forsilicon waveguides with ultra-high efficiency,” Opt. Express 23,22553-22563 (2015).

[6] Shimizu, N. (March 1984). Fusion splicing between deposited silicawaveguides and optical fibers. Electronics and Communications in Japan(Part I: Communications), 67(9), 115-122. doi:10.1002/ecja.4400670914.

[7] Pal, P., & Knox, W. H. (2008). Low loss fusion splicing of micronscale silica fibers. Optics Express, 16(15), 11568.doi:10.1364/oe.16.011568.

What is claimed is:
 1. A device for attaching at least one optical fiberto a chip comprising: at least one nanowaveguide disposed on a substrateof a chip to be attached to an at least one off-chip fiber respectively,each nanowaveguide of said at least one nanowaveguide having ananowaveguide end and being disposed in general parallel relation to alongitudinal axis extending along a length of the substrate; and anoxide taper mode converter disposed around said nanowaveguide end ofeach nanowaveguide and modally coupled to each nanowaveguide of said atleast one nanowaveguide respectively, said oxide taper mode converterextending past said nanowaveguide end and beyond said substrate of saidchip along a longitudinal axis of direction of each nanowaveguide ofsaid at least one nanowaveguide, and adapted such that eachcorresponding fiber of at least one off-chip fiber corresponds to acleaved fiber end, each cleaved fiber end to be fused to each oxidetaper mode converter respectively to optically couple and mode matcheach cleaved fiber end to each of said nanowaveguide ends of eachnanowaveguide of said at least one nanowaveguide by a modal coupling. 2.The device of claim 1, wherein said at least one off-chip fiber isoptically coupled to a connector at an end of said at least one off-chipfiber opposite to said chip.
 3. The device of claim 2, wherein a chip isprovided in a box or package, and wherein said optical connectorcomprises a plug connectable or a plug replaceable component.
 4. Thedevice of claim 1, wherein said nanowaveguide end further comprises atapered nanowaveguide end.
 5. The device of claim 1, wherein said oxidetaper mode converter comprises a horn taper or an inverse horn taper. 6.The device of claim 1, wherein said oxide taper mode converter comprisesa straight taper.
 7. The device of claim 1, wherein an oxide taper modeconverter taper comprises a selected one of: a linear taper, ahyperbolic tangent taper, an elliptical taper, an exponential taper, anda s-bend taper.
 8. The device of claim 1, wherein a portion of saidsubstrate adjacent to said at least one oxide taper mode converter isremoved to further optically isolate said oxide taper mode converterfrom said substrate.
 9. The device of claim 1, wherein at least one ofsaid at least one off-chip fiber comprises a circular cross section. 10.The device of claim 1, wherein at least one of said at least oneoff-chip fiber comprises a square or rectangular cross section.
 11. Thedevice of claim 1, wherein at least one of said at least onenanowaveguide comprises a rectangular or trapezoidal cross section. 12.The device of claim 1, wherein said cleaved fiber end further comprisesa fiber end taper.
 13. The device of claim 1, wherein each nanowaveguideend of said at least one nanowaveguide is dimensioned to extend beyondan edge segment of the substrate.
 14. The device of claim 1, whereinsaid at least one nanowaveguide and a corresponding fiber of the atleast one off-chip fiber are in general parallel relation.
 15. A devicecomprising: a nanowaveguide having a nanowaveguide end, thenanowaveguide being at least partially disposed on a substrate of achip, the nanowaveguide extending in general parallel relation to alongitudinal axis extending along the substrate, and having ananowaveguide end; and an oxide taper mode converter at least partiallydisposed over the substrate and around the nanowaveguide end of thenanowaveguide, the oxide taper mode converter extending along alongitudinal axis of direction of the nanowaveguide beyond thenanowaveguide end and beyond the substrate of the chip, the oxide tapermode converter coupled to an off-chip fiber to optically couple and modematch the off-chip fiber to the nanowaveguide.
 16. The device of claim15, wherein the off-chip fiber includes a cleaved fiber end, the cleavedfiber end fused to the oxide taper mode converter.
 17. The device ofclaim 15, including a plurality of nanowaveguides and a plurality ofoxide taper mode converters; wherein individual oxide taper modeconverters are coupled to respective individual nanowaveguides of theplurality of nanowaveguides and to respective individual off-chipfibers.
 18. The device of claim 17, wherein each individual oxide tapermode converter is at least partially disposed over the substrate andextends along the longitudinal axis of direction of respectiveindividual nanowaveguides beyond the nanowaveguide ends and beyond thesubstrate.
 19. The device of claim 18, wherein individual oxide tapermode converters are in parallel relation to respective individualnanowaveguides of the plurality of nanowaveguides and to thelongitudinal axis of the substrate.
 20. The device of claim 15, whereinthe nanowaveguide end of the nanowaveguide extends beyond the substrate.